1. Field of the Invention
The present invention generally relates to a development device and an image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction machine having at least two of these capabilities, that includes a development device.
2. Description of the Related Art
At present, one-component type development devices, which are suitable for reducing costs and mechanical size, are widely used in multicolor image forming apparatuses, and there is an increasing demand for increasing image formation speed and image quality. Accordingly, also in one-component type development devices, it is desirable to reduce toner particle size to improve image quality.
FIG. 27 illustrates a configuration of a typical multicolor image forming apparatus employing one-component development and intermediate transfer methods. Referring to FIG. 27, an image forming apparatus 600 includes image forming units 611, 612, 613, and 614 parallel to each other for forming multiple (e.g., four) different color images. Instead of the intermediate transfer method, a direct transfer method to form images directly on sheets of recording media, may be used.
The image forming units 611, 612, 613, and 614 have a similar configuration except the color of toner. In each of the image forming units 611, 612, 613, and 614, after a charging roller 622 charges a photoreceptor 621, an optical writing device exposes the photoreceptor 621, forming a latent image thereon. Then, a development device 630 develops the latent image with one-component developer (i.e., toner) into a toner image, which is transferred to an intermediate transfer belt 623. Respective color toners are superimposed one on another on the intermediate transfer belt 623, forming a multicolor toner image. Then, the toner image is transferred by a secondary transfer unit 624 onto a sheet and fixed thereon by a fixing device 625.
FIG. 28 is a cross-sectional view of the one-component type development device 630.
The development device 630 includes a developer container 631 for containing nonmagnetic one-component developer (toner), and toner is supplied to a supply roller 632 positioned in a lower portion of the development device 630 and constructed of a foamed material. Toner is further supplied from the supply roller 632 to a development roller 633 rotating in the directions indicated by arrow shown in FIG. 28. The development roller 633 may a roller having an elastic layer or a metal roller having an abraded surface. Subsequently, toner is triboelectrically charged to have a negative polarity in a nip between a doctor blade 634 and the development roller 633. Simultaneously, the amount of toner carried on the development roller 633 is adjusted. The development roller 633 rotates in contact with the photoreceptor 621 or without contacting the photoreceptor 621 and supplies toner thereto.
Small-diameter toner is typically produced through polymerization. Additionally, small-diameter toner having a particle size of 6 μm or smaller can be produced through pulverization. Small-diameter toner, however, tends to coagulate, receiving stress, and can firmly adhere to the doctor blade when coagulated toner accumulates in the nip between the doctor blade and the development roller.
Various approaches have been tried to prevent adhesion of toner. For example, JP-2008-292594-A proposes a development roller including a cylindrical or columnar base roughened to have projections and recessed in its outer circumferential surface and a surface layer covering the base and including lubricating fine particles lubricative to metal and toner. An outer circumferential surface of the surface layer is configured to reduce contact areas with toner while securing toner carrying capabilities.
The projections on the surface of the development roller is worn and abraded over time, and thus its operational life expires because the possibility of toner adhesion increases. It is desired to expand the operational life of the development roller.